This invention relates to sound attenuating materials. More particularly, this invention relates to sound attenuating materials for use in situations in which the velocity of the gas particles impinging on the sound absorbing material varies over a substantial velocity range.
In many situations it is desirable or necessary to attenuate sound waves that are travelling within a moving media. To be effective in such an application, sound attenuating materials must not only be capable of satisfactory attenuation of impinging sound waves, but must also be capable of permeation by the moving media. Such applications are often complicated by the fact that the media may be moving at a particular velocity during one period of time and may be moving at substantially different velocity during some other time period. Further, the moving media may not exhibit a uniform velocity over the region in which the sound attenuating material is located.
For example, sound attenuating material utilized to provide noise suppression in the inlet ducts of jet aircraft engines is not only subjected to the sound waves which constitute the noise, but is also subjected to high velocity of air flowing through the inlet duct. The velocity of the air is not constant but varies with such parameters as engine speed. Further, due to the construction of conventional inlet ducts, the velocity of the air impinging on one region of such a sound attenuating material may be substantially different than the velocity of the air impinging on other regions of the same sound attenuator. Additionally, such a sound attenuating material is subjected to a variety of contaminants such as particulate matter carried by the air and other external factors such as abrasive forces due to the particulate matter carried in the turbulent airflow and high level sonic stress due to the high velocity of the air travelling through the inlet duct.
The performance of a sound attenuating material under conditions in which the velocity of the gas or fluid impinging on the material varies over a substantial velocity range is often described in terms of the material non-linearity. Non-linearity in this context being the ratio of the change in specific airflow resistance to a change in the velocity of the gas or fluid impinging on the material.
In the prior art, sound attenuating materials exhibiting relatively low non-linearity have generally comprised a laminated structure including a porous woven fiberglass material or a metal structure, such as a perforated metal sheet or a woven metal mesh, that is bonded to one surface of a core material having an air impervious layer bonded to the outer surface. The core material includes a large number of cavities that are open to the acoustic energy passing through the porous covering material. Incident sound energy is partially dissipated by the porous covering material and further dissipated within the core cavities.
Although the prior art structure, especially those sound attenuators utilizing a porous metallic covering, have been partially effective, the prior art has not achieved a sound attenuating material exhibiting a sufficiently low non-linearity and a high tolerance to the deleterious effects of the surrounding environment.
Accordingly, it is an object of this invention to provide a sound attenuating material that exhibits low non-linearity.
It is another object of this invention to realize a low non-linearity sound attenuating composite that is highly resistant to contamination, abrasion, and sonic fatigue.
It is yet another object of this invention to realize a low non-linearity sound attenuating laminate that can be economically manufactured and easily fabricated as sound attenuating panels having surfaces of compound curvature.